Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 5-100 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, for example in the form of a droplet, stream or cluster of material, with a laser beam.
For this process, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include out-of-band radiation, high energy ions and debris, e.g., atoms and/or clumps/microdroplets of the target material.
These plasma formation by-products can potentially heat, damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, collector mirrors including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and/or grazing incidence mirrors, the surfaces of metrology detectors, windows used to image the plasma formation process, and laser input window(s). The heat, high energy ions and/or debris may be damaging to the optical elements in a number of ways, including coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them. Thus, it is typically desirable to minimize the amount of and/or the effect of plasma generated debris.
Heretofore, LPP systems have been disclosed in which droplets in a droplet stream are irradiated by a separate laser pulse to form a plasma from each droplet. Also, systems have been disclosed in which each droplet is sequentially illuminated by more than one light pulses. In some cases, each droplet may be exposed to a so-called “pre-pulse” and a so-called “main pulse”, however, it is to be appreciated that more than one pre-pulse may be used and more than one main pulse may be used, and that the functions of the pre-pulse and main pulse may overlap to some extent. Typically, the pre-pulse(s) may affect some or all of the target material to heat, expand, gasify, vaporize, ionize, generate a weak plasma and/or generate a strong plasma, and the main pulse(s) may function to convert most or all of the pre-pulse affected material into plasma and thereby produce an EUV light emission. In some cases, pre-pulsing may increase the efficiency of the material/pulse interaction due to a larger cross-section of material that is exposed to the main pulse, a greater penetration of the main pulse into the material due to the material's decreased density, or both. Another benefit of pre-pulsing is that it may expand the target to the size of the focused main pulse, allowing all of the main pulse to participate. This may be especially beneficial if relatively small droplets are used as targets and the irradiating light cannot be focused to the size of the small droplet. Thus, in some applications, it may be desirable to use pre-pulsing to increase conversion efficiency and/or allow use of relatively small, e.g., so-called, mass limited targets. The use of relatively small targets, in turn, may be used to lower debris generation and/or reduce source material consumption.
With the above in mind, it may be desirable to use a specific pre-pulse energy to irradiate the target material. Several factors may affect the selection of this target pre-pulse energy including the size of the target material droplet and corresponding pre-pulse focal spot, the level of accuracy that is achievable in targeting the droplet with the pre-pulse, the pre-pulse pulse duration, the pre-pulse wavelength, the desired level of EUV output energy, EUV conversion efficiency, and prepulse and/or main pulse peak intensity.
As indicated above, one technique to produce EUV light involves irradiating a target material. In this regard, CO2 lasers, e.g., outputting light at infra-red wavelengths, e.g. wavelengths in the range of about 9.2 μm to 11.2 μm, may present certain advantages as a drive laser irradiating a target material in an LPP process. This may be especially true for certain target materials, e.g., materials containing tin. For example, one advantage may include the ability to produce a relatively high conversion efficiency between the drive laser input power and the output EUV power. Another advantage of CO2 drive lasers may include the ability of the relatively long wavelength light (for example, as compared to deep UV at 193 nm) to reflect from relatively rough surfaces such as a reflective optic that has been coated with tin debris. This property of 10.6 μm radiation may allow reflective mirrors to be employed near the plasma for, e.g., steering, focusing and/or adjusting the focal power of the drive laser beam.
In some cases, it may be desirable to employ a MoPa (Master Oscillator-Power Amplifier) arrangement to produce the relatively high power main pulses used in the LPP process. In this case, it may also be advantageous in certain circumstances to use some or all of the main pulse amplifier to amplify pre-pulses from a pre-pulse seed laser. In this case, it may be desirable to use a pre-pulse wavelength that does not substantially reduce the amplifier gain for the main-pulse wavelength. Other factors can effect the selection of wavelength for the main pulse and pre-pulse. For example, it is typically desired to use a main pulse wavelength that will produce the greatest amount of energy. Also, when lenses are used to focus the pulses, the amount of chromatic aberration that is tolerable may affect the selection of the main pulse and pre-pulse wavelengths. In addition, the use of dichroic beam splitters/combiners may also introduce limitations on main pulse/pre-pulse wavelength selection.
Lastly, other factors, such as the desire to use pre-pulses having a relatively large pulse rise-time may dictate the type of pre-pulse seed laser used (e.g. the gain media parameters, discharge type, optical cavity, etc.). Some types of seed lasers may only be operated to produce seed laser output pulse energies within a limited output energy range. For example, a range of pulse energies may exist outside of which the laser is unstable in that it may not be operated to produce consistent, repeatable laser parameters such as pulse energy. Although an attenuator may be positioned downstream of a seed laser to expand its operational range in some cases, the use of an attenuator can cause undesirable complications and may unnecessarily waste energy. In some cases, a range of pulse energies may exist outside of which suitable optics such as metrology detectors may not be available. These limitation of seed output pulse energy may, in turn, affect the selection of pre-pulse wavelength needed to produce the desired pre-pulse target energy at the droplet after amplification, since amplifier gain is dependent on seed pulse wavelength.
With the above in mind, Applicants disclose a Master Oscillator-Power Amplifier Drive Laser with Pre-Pulse for EUV Light Source.